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Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Sep. 30, 2020
--- | --- | ---
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 13,051,199 | $ 12,959,272
Commercial loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 7,341,847 | 7,044,426
Commercial loans | Multi-family |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 1,609,800 | 1,538,240
Commercial loans | Commercial real estate |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 1,942,854 | 1,884,688
Commercial loans | Commercial and Industrial [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 2,243,465 | 2,115,513
Commercial loans | Construction |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 1,393,107 | 1,352,414
Commercial loans | Land - acquisition & development |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 152,621 | 153,571
Consumer loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 5,709,352 | 5,914,846
Consumer loans | Construction |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 299,351 | 295,953
Consumer loans | Single-family residential |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 5,048,435 | 5,293,962
Consumer loans | Land - consumer lot loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 109,845 | 101,394
Consumer loans | HELOC |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 140,272 | 140,222
Consumer loans | Consumer |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | 111,449 | 83,315
Performing Loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 12,992,793 | $ 12,930,216
% of Total Loans | 99.60% | 99.80%
Performing Loans | Commercial loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 7,308,619 | $ 7,038,657
% of Total Loans | 99.50% | 99.90%
Performing Loans | Commercial loans | Multi-family |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 1,609,800 | $ 1,538,240
% of Total Loans | 100.00% | 100.00%
Performing Loans | Commercial loans | Commercial real estate |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 1,911,457 | $ 1,880,917
% of Total Loans | 98.40% | 99.80%
Performing Loans | Commercial loans | Commercial and Industrial [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 2,242,871 | $ 2,115,184
% of Total Loans | 100.00% | 100.00%
Performing Loans | Commercial loans | Construction |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 1,391,870 | $ 1,350,745
% of Total Loans | 99.90% | 99.90%
Performing Loans | Commercial loans | Land - acquisition & development |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 152,621 | $ 153,571
% of Total Loans | 100.00% | 100.00%
Performing Loans | Consumer loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 5,684,174 | $ 5,891,559
% of Total Loans | 99.60% | 99.60%
Performing Loans | Consumer loans | Construction |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 299,351 | $ 295,953
% of Total Loans | 100.00% | 100.00%
Performing Loans | Consumer loans | Single-family residential |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 5,024,086 | $ 5,271,531
% of Total Loans | 99.50% | 99.60%
Performing Loans | Consumer loans | Land - consumer lot loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 109,402 | $ 101,151
% of Total Loans | 99.60% | 99.80%
Performing Loans | Consumer loans | HELOC |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 139,938 | $ 139,669
% of Total Loans | 99.80% | 99.60%
Performing Loans | Consumer loans | Consumer |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 111,397 | $ 83,255
% of Total Loans | 100.00% | 99.90%
Non-Performing Loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 58,406 | $ 29,056
% of Total Loans | 0.40% | 0.20%
Non-Performing Loans | Commercial loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 33,228 | $ 5,769
% of Total Loans | 0.50% | 0.10%
Non-Performing Loans | Commercial loans | Multi-family |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 0 | $ 0
% of Total Loans | 0.00% | 0.00%
Non-Performing Loans | Commercial loans | Commercial real estate |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 31,397 | $ 3,771
% of Total Loans | 1.60% | 0.20%
Non-Performing Loans | Commercial loans | Commercial and Industrial [Member] |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 594 | $ 329
% of Total Loans | 0.00% | 0.00%
Non-Performing Loans | Commercial loans | Construction |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 1,237 | $ 1,669
% of Total Loans | 0.10% | 0.10%
Non-Performing Loans | Commercial loans | Land - acquisition & development |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 0 | $ 0
% of Total Loans | 0.00% | 0.00%
Non-Performing Loans | Consumer loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 25,178 | $ 23,287
% of Total Loans | 0.40% | 0.40%
Non-Performing Loans | Consumer loans | Construction |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 0 | $ 0
% of Total Loans | 0.00% | 0.00%
Non-Performing Loans | Consumer loans | Single-family residential |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 24,349 | $ 22,431
% of Total Loans | 0.50% | 0.40%
Non-Performing Loans | Consumer loans | Land - consumer lot loans |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 443 | $ 243
% of Total Loans | 0.40% | 0.20%
Non-Performing Loans | Consumer loans | HELOC |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 334 | $ 553
% of Total Loans | 0.20% | 0.40%
Non-Performing Loans | Consumer loans | Consumer |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |
Amount | $ 52 | $ 60
% of Total Loans | 0.00% | 0.10%

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